1. Field of the Invention
The present invention relates generally to the field of encoder apparatus, and more particularly to an electronic circuit that simulates a mechanical encoder.
2. Description of Related Art
Mechanical encoder apparatuses (a.k a., optical or incremental encoders) are extensively used in speed control and counting applications for industrial machines or assembly lines. The encoders convert shaft rotation from a master machine or process into a proportional electronic output, commonly square wave pulses, that provide an accurate means of gauging velocity and direction. In addition, some encoders also provide position output signals. The output signals from the encoder are then used to control or drive a slave device or machine. Encoders are specified as having a particular number of pulses per revolution (PPR). Specific PPR ratings are often required to correctly translate the speed of the master device or process into a desired speed for the driven or slave device.
A traditional mechanical encoder comprises a rotary shaft that is driven, for example, by a machine or assembly line whose speed or position is to be monitored. The rotary shaft includes a disk having one or more slits or apertures formed therein and spaced in an annular fashion about the disk. A light emitting device, such as a light emitting diode (LED), and a light detecting device, such as a photodiode, are positioned in alignment with one another on opposing sides of the disk. As the machine turns the rotary shaft, light is intermittently transmitted to the photodiode as the apertures are rotated into position allowing passage of the light. These light pulses received by the photodiode can then be converted into electrical waveforms and electronically processed to calculate the speed of the machine, determine the position of a machine component, increment a counter, or drive a slave device, for example.
While mechanical encoders of the type just described are useful for their intended purposes, they do have their drawbacks. As the mechanical rotary encoder is an electromechanical device, it is susceptible to failure unless properly maintained. In addition, mechanical encoders are generally used in conjunction with devices known as rate multipliers. Rate multipliers are used when the PPR of the encoder is not in the specified working range required by the position sensing device, counter, slave device, etc., and cannot be corrected by using mechanical gearing methods to obtain the proper number of revolutions. Mechanical gearing of a traditional encoder is often not feasible or desirable because of cost and/or complexity and the inherent lash and instability in any mechanical system. Moreover, existing encoders and rate multipliers do not offer the flexibility of generating a quadrature pulse train (i.e., two pulse trains in which a second pulse train is derived from a first pulse train by introducing a 90.degree. phase shift) in which the pulse width and frequency can be precisely and easily controlled without substituting mechanical components. Such precision is particularly desirable, if not required, for applications such as servo motor control. A quadrature pulse train is preferred because it can convey direction in addition to speed information. For example, the movement direction of a machine or conveyor can be indicated by the phase shift between two signals in the quadrature pulse train.
Thus, what is sought after is a highly reliable, low maintenance electronic device that can generate a tunable, quadrature pulse train in response to an input signal representative of the speed of a machine, process, assembly line or device and can be used in place of a traditional mechanical encoder. The signals comprising the quadrature pulse train can then be used to control or drive other slave machines, processes, assembly lines or devices that use the speed and/or direction of the original machine or assembly line as a reference.